This application claims priority from Korean Patent Application No. 2002-44631, filed on Jul. 29, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to carbon nanotubes, and more particularly, to carbon nanotubes for fuel cells, which are directly grown over a carbon substrate and whose internal and external walls are uniformly doped with metallic catalyst particles, and a method for fabricating the carbon nanotubes, and a fuel cell using the carbon nanotubes for an electrode.
2. Description of the Related Art
Recently, with growing concerns about the environment and the exhaustion of energy resources, and the commercialization of fuel cell automobiles, there is an urgent need for the development of reliable, high-performance fuel cells that are operatable at an ambient temperature with high energy efficiency and for the development of polymer membranes capable of increasing the efficiency of fuel cells.
Fuel cells are new power generating systems that convert energy produced through the electrochemical reactions of fuel and oxidative gas directly into electric energy. Such fuel cells can be categorized into fuel cells with molten carbonate salt, which are operable at a high temperature of 500-700° C., fuel cells with phosphoric acid, which are operable around 200° C., alkaline electrolyte fuel cells operable between room temperature and 100° C., and solid polymer electrolyte (SPE) fuel cells operable at a temperature of ambient ˜100° C.
SPE fuel cells include proton-exchange membrane fuel cells (PEMFCs) using hydrogen gas as a fuel source and direct methanol fuel cells (DMFCs) which generate power using liquid methanol solution directly applied to the anode as a fuel source.
SPE fuel cells, which are emerging as the next generation of a clean energy source alternative to fossil fuels, have high power density and high-energy conversion efficiency. In addition, SPE fuel cells are workable at an ambient temperature and are easy to hermetically seal and miniaturize, so they can be extensively applied to the fields of pollution-free vehicles, power generating systems for house use, mobile telecommunications equipment, medical equipment, military equipment, equipment in space, and the like.
The basic structure of a PEMFC as a power generator producing a direct current through the electrochemical reaction of hydrogen and oxygen is shown in FIG. 1. Referring to FIG. 1, a PEMFC has a proton-exchange membrane 11 interposed between an anode and a cathode. The proton-exchange membrane 11 is formed of an SPE with a thickness of 50-200 μm. The anode and cathode includes anode and cathode backing layers 14 and 15, respectively, for supplying reaction gases or liquid, and catalyst layers 12 and 13, respectively, where oxidation/reduction of reaction occur, forming catalyst electrodes (hereinafter, the anode and cathode will be referred to as “catalyst electrodes”). In FIG. 1, reference numeral 16 denotes a carbon sheet having gas injection holes and acting as a current collector.
As hydrogen as a reaction gas is supplied to a PEMFC having the structure as described above, hydrogen molecules decompose into protons and electrons through oxidation reaction in the anode. These protons reach the cathode through the proton-exchange membrane 11. Meanwhile, in the cathode, oxygen molecules take electrons from the anode and are reduced to oxygen ions through reaction. These oxygen ions react with the protons from the anode to produce water.
As shown in FIG. 1, in the gas diffusion electrodes of the PEMFC, the catalyst layers 12 and 13 are formed on the anode and cathode backing layers 14 and 15, respectively. The anode and cathode backing layers 14 and 15 are formed of carbon cloth or carbon paper. The surfaces of the anode and cathode backing layers 14 and 15 are treated for reaction gases and water to easily permeate into the proton-exchange membrane 11 before and after reaction.
DMFCs have a similar structure to the PEMFC described above, but use liquid methanol solution instead of hydrogen as a fuel source. As methanol solution is supplied to the anode, an oxidation reaction occurs in the presence of a catalyst to generate protons, electrons, and carbon dioxide. Although DMFCs has lower energy efficiency than PEMFCs, the use of a liquid fuel in DMFCs makes their application to portable electronic devices easier.
Electrodes, fuel, electrolyte membranes for high energy density, high power fuel cells have been actively researched. In addition, there have been attempts to increase the activity of a catalyst used in the electrodes. Since the activity of catalysts is proportional to the reaction surface area thereof, it is necessary to increase the reaction surface area by reducing the diameter of catalyst particles to a few nanometers and to uniformly distribute such nano-sized catalyst particles over the electrodes.
Conventionally, catalysts such as platinum have been applied as paste, uniformly to electrode backing layers of porous carbon substrate. However, the dispersion of the catalyst in the electrode backing layers is not uniform, and the surface area of the carbon carrier and electrical conductivity are not large enough.
Japanese Laid-open Publication No. 2000-63112 discloses a method for manufacturing single-walled carbon nanotubes into which trace of metal is incorporated by CO2 laser irradiation of a metal containing carbon source. In this method, the use of laser limits the area where carbon nanotubes are grown. To be applied to fuel cells, an additional step of coating electrodes with a paste of the carbon nanotubes is required, which makes the overall fuel cell manufacturing process complicated.